With regard to the recent digital transmission, for example, the error correcting method, which uses the concatenated code described in ITU-T Recommendation G.975.1 (ITU-T recommendation G series) I.3, is proposed.
For example, ITU-T Recommendation G.709 (ITU-T recommendation G series) describes the OTU (Optical channel Transport Unit) frame as a transmission frame which is used in a high-speed and large-volume optical communication system installing an error correction function. The OTU frame includes an area for a payload which stores communication data, an area for a frame synchronization, an area for maintenance and monitoring information, an area for mapping the payload, and an area which stores information on an error correcting code for correcting a bit error.
FIG. 7 shows a configuration of an error correcting apparatus according to a related art. An error correcting apparatus 101 includes a sending unit 102 and a receiving unit 103. A transmission signal, which is sent into a communication line 4 by the sending unit, is received by the receiving unit 103.
According to the forward error correction (FEC) scheme described in ITU-T Recommendation G.975.1, a code, in which an outer code and an inner code are concatenated to each other, is used, and an interleaver 105 and a deinterleaver 106 are arranged between a coder (outer code) 109 and a coder (inner code) 107, and between a decoder (outer code) 110 and a decoder (inner code) 108 respectively, as shown in FIG. 7. The concatenated code is applied to the repeated decoding.
FIG. 8 shows a configuration of digital data which is processed with an error correcting method according to the related art. Data per one row in FIG. 8 is sent and received at a predetermined timing. An error correcting process for the outer code is carried out to a data stream which is sent and received at each timing. In contrast, an error correcting process for the inner code is carried out to data, which is extracted through the interleaver from a predetermined position in a past data stream.
Though an arbitrary code may be employed for the inner/outer code, an example with the block code will be described in the present application
A node apparatus disclosed in Japanese Patent Application Laid-Open No. 2010-166378 transmits a transmission frame which is generated through adding an overhead and an error correcting code to information data. Moreover, a repeated decoding process for decoding the error correcting code is carried out for the received transmission frame. When all error bits of the transmission frame are corrected before a predetermined number of times of the repeated decoding processes are completed, the error correcting process afterward is stopped. As a result, unnecessary power consumption is suppressed.
The error correcting method, which is based on the concatenated code by use of the inner code and the outer code and which is described in ITU-T Recommendation G.975.1 and Japanese Patent Application Laid-Open No. 2010-166378, has the following problem.
Firstly, the concatenation of the error correcting codes lowers the error correcting ability. While information data of the inner code composes not only data of the inner code but also data of the outer code, redundant data of the inner code does not compose the outer code. For this reason, the error correction is carried out only to the inner code. According to FIG. 8, since redundant data of the outer code is included in the information data of the inner code, in the case that a transmission failure causes an error in the redundant data of the outer code, the error is corrected in the error correcting process for the inner code. Even when the inner code could not carry out the error correction, the error can be corrected by the outer code error correcting process. On the other hand, only the inner code error correcting process is carried out for the redundant data of the inner code, and the outer code error correcting process is not carried out. For this reason, in the case that the redundant code of the inner code is caused an error, there is a fear that the error correcting process for the inner code is not carried out correctly.
Secondly, the error correcting method using the concatenated code increases a circuit scale. That is, since a code length of the outer code and a code length of the inner code are different from each other, the coder and the decoder, which are used in the error correcting process for the inner code, are different respectively from the coder and the decoder used in the error correcting process for the outer code. Consequently, two circuits are needed in each of the error correcting process for the inner code, and the error correcting process for the outer code.
Meanwhile, as digital transmission data is being enlarged in volume, and speed of the digital transmission data is increasing, it is required to improve the error correcting ability in the error correcting process the transmission data. Moreover, it is required to make a processing circuit, which carries out the data error correction, smaller in a circuit scale in response to a request for miniaturizing a transmission apparatus and lowering the cost of the transmission apparatus.